This invention relates generally to superconducting electrical generators and more particularly to superconducting electrical generators including means for actively cooling the thermal radiation shield disposed between the electromagnetic shield and the field windings of the rotor.
One type of superconducting generator includes a rotating field winding maintained at superconducting temperatures and a stationary winding at the temperature of the surrounds. The rotor winding is contained in a permanently sealed vacuum enclosure including a cylindrical torque tube. A cylindrical electromagnetic shield surrounds the rotor and serves to screen the superconducting winding from transient magnetic fields and non-synchronous components of magnetic fields produced by unbalanced stator currents. The electromagnetic shield is generally at the temperature of the surrounds. In the prior art a thermal radiation shield has been interposed between the electromagnetic shield and the winding region of the rotor to receive the radiation from the electromagnetic shield and re-radiate to the windings at a lower temperature. It is known that the thermal radiation reaching the winding region of the rotor is dependent upon the fourth power of the temperature of the thermal radiation shield. It is therefore important to maintain the shield at uniform low temperature so that radiation to the windings is limited. The thermal radiation shield is generally attached to the torque tube extension on either end of the machine at a location having a temperature between ambient and the winding temperature.
In a large superconducting generator, a substantial heat load is input to the thermal radiation shield both due to thermal radiation and due to electromagnetic fields caused for example, by negative sequence currents in the armature. The thickness of the shield required to conduct this heat to its cold ends can be substantial and can significantly reduce the generator power density and stability. The time required to cool a thermal radiation shield to its operating temperature from ambient temperature can be considerable if the heat must be conducted axially to the ends of the shield.
The thermal radiation shield can be cooled by a helium vapor stream in the same manner that torque tube and and windings are cooled. However, this can introduce significant plumbing and flow control problems.